This invention relates generally to endoluminal grafts or xe2x80x9cstentsxe2x80x9d and, more specifically, to stent delivery systems or xe2x80x9cintroducersxe2x80x9d.
A stent is an elongated device used to support an intraluminal wall. In the case of a stenosis, a stent provides an unobstructed conduit for blood in the area of the stenosis. Such a stent may also have a prosthetic graft layer of fabric or covering lining the inside and/or outside thereof. Such a covered stent is commonly referred to in the art as an intraluminal prosthesis, an endoluminal or endovascular graft (EVG), or a stent-graft. As used herein, however, the term xe2x80x9cstentxe2x80x9d is a shorthand reference referring to a covered or uncovered such stent.
A covered stent may be used, for example, to treat a vascular aneurysm by removing the pressure on a weakened part of an artery so as to reduce the risk of rupture. Typically, a stent is implanted in a blood vessel at the site of a stenosis or aneurysm endoluminally, i.e. by so-called xe2x80x9cminimally invasive techniquesxe2x80x9d in which the stent, restrained in a radially compressed configuration by a sheath or catheter, is delivered by a stent deployment system or xe2x80x9cintroducerxe2x80x9d to the site where it is required. The introducer may enter the body through the patient""s skin, or by a xe2x80x9ccut downxe2x80x9d technique in which the entry blood vessel is exposed by minor surgical means. When the introducer has been threaded into the body lumen to the stent deployment location, the introducer is manipulated to cause the stent to be ejected from the surrounding sheath or catheter in which it is restrained (or alternatively the surrounding sheath or catheter is retracted from the stent), whereupon the stent expands to a predetermined diameter at the deployment location, and the introducer is withdrawn. Stent expansion may be effected by spring elasticity, balloon expansion, or by the self-expansion of a thermally or stress-induced return of a memory material to a pre-conditioned expanded configuration.
Referring now to a typical prior art stent introducer as seen in FIG. 1A and FIG. 1B, there is shown a standard pre-loaded stent delivery system 10 comprising an outer sheath 12, a compressed stent 14 loaded therein, and a conventional stabilizer 16 loaded adjacent to the proximal end 17 of the stent. As used herein, the term xe2x80x9cproximalxe2x80x9d refers to the end closer to an access location outside the body whereas xe2x80x9cdistalxe2x80x9d refers to the farther from the access location. The term xe2x80x9cstabilizerxe2x80x9d is used in the art to describe component 16 of stent delivery systems used to stabilize or prevent retraction of stent 14 when sheath 12 is retracted, thus effecting deployment of the stent into a desired location by forcing relative movement between the sheath and the stent.
Delivery system 10 also may comprise a catheter tip 20 at its distal end attached to an internal sheath 23 that runs through the delivery system through inner lumen 22 in stabilizer 16, as shown in FIG. 1A. A stabilizer handle 26 is typically located at the proximal end of stabilizer 16, outside the body lumen. Internal sheath 23 may guide the delivery system through the body lumen over a guidewire (not shown) to the area to be repaired, or may be adapted for inflating a balloon (if applicable), and/or for flushing the system. The delivery system may additionally have radiopaque markers (not shown) at selected locations therein to be used for fluoroscopic guidance of the system through the body lumen.
To deploy stent 14, delivery system 10 is threaded through the body lumen to a desired location for stent deployment. Outer sheath 12 is then retracted, and stabilizer 16 acts as a stabilizer to keep stent 14 from retracting with the sheath. As outer sheath 12 retracts, stent 14 is exposed and expands into place in the body lumen to be repaired.
Some stents have relatively low column strength either along their whole length or in discrete sections thereof. Their low column strength may be an inherent result of a flexible stent architecture. Such low-column-strength stents or stent sections are easily deformed in a longitudinal direction, and thus longitudinal force is not transmitted along the length of the stent. This inability to transmit longitudinal force may result in such stents collapsing in an accordion fashion as the sheath is retracted or as the stent is ejected by movement of the stabilizer, when the stent is deployed using a standard stabilizer positioned at the proximal end of the stent. This collapsing is caused primarily by frictional forces, such as frictional forces between the sheath and the stent (in the case where the stent is deployed by retraction of the sheath) or between the stent and the body lumen (in the case where the stent is deployed by ejection). Thus, a low column strength segment is one which tends to collapse due to frictional forces upon deployment of the stent by a conventional stabilizer positioned at the proximal end of the stent. This collapsing may cause damage to the stent or incorrect deployment. Thus, it is desirable to employ a stent-stabilizer combination that avoids such collapse.
U.S. Pat. No. 5,702,418 to Ravenscroft, of common assignment with the present invention, discloses an introducer comprising a stabilizer having an inner core that underlies a compressed stent within a sheath. The core has one or two proximal rings attached to and extending radially from the surface of the inner core for engaging the compressed stent at the proximal end thereof. Ravenscroft further describes but does not illustrate stabilizer embodiments having additional rings, rings including slots for receiving portions of the stent overlying the rings, and rings formed or defined by a plurality of protuberances or fingers extending from the core to engage and interlock the stent minimum inner diameter at the proximal end thereof. The purpose of these rings, according to Ravenscroft, is to allow selective retraction and deployment of the stent.
Thus, it is known to have rings or protuberances that engage the inner diameter of the stent, but only with respect to one or more rings that engage the proximal end of the stent to enable selective retraction and deployment of the stent. There remains a need, therefore, for a means to facilitate deployment of endoluminal stents with relatively low column strength.
In accordance with this invention, there is provided a stent delivery system for receiving, endoluminally transporting, and endoluminally deploying an elongated stent for holding open a body lumen, which system facilitates the use of stents with low column strength. The stent delivery system comprises a stent, an overlying sheath, and a stabilizer. The stent has an inner periphery that defines an interior space extending lengthwise along at least a part of the stent, at least one longitudinal segment of which may comprise relatively low column strength (or reduced column strength as compared to other parts of the stent), in that such segment is easily collapsed longitudinally. Such a low column strength segment may comprise all or nearly all the length of the stent. The stent is adapted to be radially compressed and loaded within the delivery system for introduction into the body lumen and expanded for deployment within the body lumen. The sheath overlies the compressed stent during introduction of the stent within the body lumen from a proximal access location to a distal deployment location. The stabilizer is disposed within the stent interior space and has at least one surface element adapted to engage the stent inner periphery in a region containing the low-column-strength segment.
The stent may comprise a plurality of peripheral members disposed in succession along the length of the stent (i.e. longitudinally), in which case the stabilizer comprises at least one surface element adapted to engage individual peripheral elements in a manner capable of imparting a longitudinal force thereto. The stabilizer may comprise a plurality of protuberances positioned peripherally about the stabilizer such that the stabilizer engages the peripheral elements in a plurality of peripheral locations. The engagement between the protuberance and the peripheral element may be a frictional engagement, or a direct mechanical engagement, for example where the protuberance penetrates an area of open space between peripheral elements of the stent.
The stabilizer typically comprises a surface element comprising one or more frictional surface areas, protuberances, or protrusions axially spaced along the stabilizer underlying the stent from a distal end to a proximal end of the low-column-strength segment, which may comprise the entire stent. The stabilizer may further comprise an inner core wherein the surface element is a sleeve or coating about the inner core. The surface element may further comprise radial protuberances in the form of rings about the inner core. The rings may be of various cross-sections, such as rectangular or triangular, may have varying lengths in one section of the stabilizer relative to another, and may have spaces of various sizes between adjacent rings. The rings may be locking rings that further comprise protrusions that penetrate into the open space between peripheral stent elements. Instead of rings, the protuberances may instead be discrete barbs, bumps, or inflatable knobs that may be arranged in a ringed configuration about the stabilizer, or may be axially and peripherally spaced in a helical pattern.
Alternatively, the stabilizer may comprise an inner core and a heat-moldable compression sleeve surrounding the inner core, the heat-moldable compression sleeve having an outer surface comprising a plurality of surface elements defined by a thermal imprint of the stent inner periphery on the compression sleeve outer surface. The invention also comprises a corresponding method for loading a stent into the stent delivery system described above. The method comprises inserting the heat-moldable portion of the stabilizer within the stent interior space, compressing the stent so that the outer surface of the heat-moldable portion is in contact with the stent inner periphery, inserting the stent and underlying stabilizer within the outer sheath, and heating the stent delivery system to thermally imprint the heat-moldable portion outer surface with an uneven topography conforming to the stent inner periphery.
The stabilizer may instead comprise about its inner core an injection-molded sleeve having a similar structure to that described. In such an embodiment, the method for loading the stent comprises radially compressing and loading the stent inside the sheath with the stabilizer inner core axially disposed within the stent interior space, and creating a sleeve over said inner core by injecting a thermoplastic material around the inner core to fill the interior space. The resulting injection-molded sleeve has an outer surface with an uneven topography conforming to the stent inner periphery.
The invention also comprises a method of delivering a stent using a stent delivery system as described herein, the method comprising urging the stent delivery system through the patient""s body to a desired deployment location and displacing the sheath longitudinally relative to the stabilizer so that the protuberances engage the stent to displace the stent relative to the sheath.